1. Field of the Disclosure
The present disclosure relates to an ophthalmological measuring device and an ophthalmological measurement method for determining geometric structures of an eye. In particular, the present invention relates to an ophthalmological measuring device and an ophthalmological measurement method for determining geometric structures of an eye, in which use is made of an optical, triangulating first measurement system and an optical, interferometric second measurement system.
2. Related Art
The laid-open specification DE 10 2007 017 599 describes an ophthalmological measurement system and an ophthalmological measurement method for determining geometric parameters in the front eye section, which parameters are required for calculating the refraction of intraocular lenses. The measurement system according to DE 10 2007 017 599 comprises, combined in a single instrument, a first measuring device for determining the axial length and a second measuring device for registering a plurality of structures in the front section such as the cornea, anterior chamber and lens. Here, the first measuring device comprises a known optical coherence interferometer (PCI=partial coherence interferometry, OCT=optical coherence tomography) operating in the time or frequency domain. The second measuring device comprises a slit illumination unit and an imaging unit for carrying out a known slice-image method. The measurement system moreover comprises a control unit designed to evaluate both the measured values required to measure the axial length and the angular and distance conditions required for a triangulation on the eye, with sections, radii and/or angles of refraction being determined from the slice images of the front eye sections supplied by the imaging unit.
The advantage of triangulating measuring devices with a slice-image method compared to those based on one or more individual beams is that triangulating slice-image methods do not generate any, or only generate few, movement artifacts caused by eye movements and thus have a comparatively higher geometric precision with the same number of measurement points. Furthermore, in the Scheimpflug configuration there is the additional advantage of allowing the depth measurement range at high optical resolution to be designed to be significantly greater than in interferometric methods. This holds true for both the lateral resolution and the depth resolution. In triangulation methods and in interferometric methods with a single scanned beam, the movement artifacts caused by eye movements generate measurement errors, which have to be corrected by an additional method that registers the eye movements. In order to be effective, such a method, not known previously, would have to be significantly faster and more precise than the measurement errors caused by the eye movements. In turn, triangulation methods with many individual beams or with structured illumination are disadvantageous in that the number of measurement points is limited and, moreover, the depth measurement range is severely limited because the imaging unit cannot be arranged in Scheimpflug configuration for all beams. The known triangulating measuring devices generally are disadvantageous in that the measurement region thereof that can be registered is limited to the front eye region (anterior chamber region), which can be illuminated directly, and the posterior chamber of the eye is not visible. In the process, the measurement region of the known triangulating measuring devices is further limited by opacity, e.g. in the cornea, by implants, e.g. lenses placed in front, and by the iris, and certain partial regions, such as the anterior chamber angle, cannot, in general terms, be registered by the known triangulating measuring devices due to tissue properties.
The advantage of known optical coherence interferometers is that the measurement region thereof that can be registered is not limited to the front eye region (anterior-chamber region) because the posterior chamber of the eye can also be measured due to the axial operating principle of these interferometers. In conventional interferometric methods, use is typically made of a scanning beam. In the process, the beam at a particular position generates a depth scan (A-scan) at this location. Relative displacement of measurement object and light beam, or scanning the beam, extends the depth scan over a contiguous surface along a scan line (B-scan). A three-dimensional scan (C-scan) is assembled from a plurality of B-scans. Thus, as soon as the interferometric methods are used for the structural measurement at a plurality of positions, eye movements generate movement artifacts and hence measurement errors. Moreover, compared to triangulating measurement methods in Scheimpflug configuration, there is the additional disadvantage that the depth measurement range of interferometric methods has to be designed to be significantly smaller at a high lateral resolution. Since the focal depth region decreases disproportionately with image resolution for optical reasons, the desire for high lateral resolution generally significantly restricts the depth measurement range, particularly in interferometric methods.
At this juncture, reference is additionally made to the fact that there are further interferometric (OCT-) methods, which can simultaneously register a plurality of A-scans (with a plurality of light beams) by means of optical parallelization. Furthermore, there are en-face techniques, which successively scan a plurality of planes at depth in order to subsequently synthesize A- and B-scans. Some of these techniques can be used to reduce movement artifacts. However, disadvantages of these techniques include the high technical and financial expenditure required for this, a reduced resolution and/or compromises (limitations) in the depth measurement range. It can be observed that there are no commercially available systems with movement- and artifact-free interferometric methods. [Drexler] provides a good overview of the prior art.
Even if triangulating methods and interferometric (OCT-) methods can be used for the geometric measurement technique, it should be noted here that they are based on different effective principles. While the operational principle of triangulating methods is based on diffuse scattering in the tissue and on surfaces in a large spatial area, interferometric methods operate with backward reflection in respect of the projecting optical system. Hence both methods supply different items of information, and thus overall more information, relating to the structural properties of the measurement object in addition to the geometry thereof.